The aims of the present research project are: a) to investigate the interactions between protons and Ca ions on the healing-over process of cardiac muscle; b) to investigate if the sealing of cardiac fibers is reversed by decreasing or removing Ca or H ions from the extracellular fluid. The influence of the extracellular Ca concentration on the rate of healing will be measured at normal pHo - 7.3 and also in acid solutions (pHo - 6.5 or 6) which are not able by themselves to promote healing-over. Measurements of membrane potential and input resistance will be made at different distances from the cut-end. The experimental values of input resistance obtained near the cut-end will be compared with theoretical values for a sealed cable or short-circuit cable. Considering that ischemia is accompanied by a lowering of both intracellular and extracellular pH and by a rise in intracellular Ca concentration, it is extremely important to know the effects of H ions on the healing process and their interactions with Ca. The question, whether the healing process of heart muscle is reversible, has important implications to cardiology.